1. Field of the Invention
The present invention relates to a coding method in a wireless mobile communication system. More particularly, the present invention relates to a Convolutional Turbo Coding (CTC) method and a device for implementing the method.
2. Description of Related Art
Mobile Worldwide Interoperability for Microwave Access (WiMAX) is a broadband access technique for implementing “last kilometer” access by using a wireless mode, instead of using a wired mode. It integrates the mobile devices with a fixed broadband network, and provides a convenient and high-speed mobile broadband connection by employing a broadband wireless access technique and a flexible network structure. The WiMAX technique is based on the Institute of Electrical and Electronics Engineers (IEEE) 802.16 standards, which are proposed for microwave and millimeter-wave frequency bands. The mobile WiMAX standard was proposed after the IEEE 802.16d fixed WiMAX standard was proposed. The mobile WiMAX aims to support mobility of the broadband access by building on research started during the standardization of the fixed WiMAX standard. Convolutional Turbo Code (CTC) is a class of Turbo code using several convolution coding schemes. The CTC is incorporated into the IEEE 802.16 and Digital Video Broadcasting—Return Channel via Satellite (DVB-RCS) standards because of its high error correction performance.
FIG. 1 illustrates a CTC encoder according to the related art. Referring to FIG. 1, the CTC encoder may comprise a ⅓ CTC encoder 101, an interleaver 102 and a puncturing unit 103. As shown in FIG. 1, input information bits are input to the ⅓ CTC encoder 101. Here, the number of encoded output information bits and parity bits is three times the number of information bits. The encoded data is then interleaved by the interleaver 102. The puncturing unit 103 punctures the interleaved data based on the transmission rate, i.e., it chooses the data bits to be transmitted and obtains the encoded bit sequence so as to complete the encoding process.
More specifically, in the ⅓ CTC encoder 101, a duo binary Circular Recursive Systematic Convolutional (CRSC) code is employed. As shown in FIG. 1, the ⅓ CTC encoder 101 may comprise a CTC interleaver 105 and a constituent encoder 104. Here, the inputs A and B to the CTC interleaver 105 represent the input information bits, which are encoded twice. First, the duo binary CRSC coding is performed on the information bits A and B. That is, a set of information bits A, and B, are simultaneously input to the constituent encoder 104 for encoding, and parity sequences Y1 and W1 are obtained. The information bits A and B are also interleaved by the CTC interleaver 105. The second constituent encoding process is then performed to the interleaved sequences. That is, interleaved information bits Aj and Bj are simultaneously input to the constituent encoder 104 so as to obtain parity sequences Y2 and W2. Each code block input into the constituent encoder 104 contains k information bits or N pairs of information bits, i.e., k=2×N, where k is a multiple of 8, and N is a multiple of 4, and 32≦N≦4096.
As shown in block 106, the interleaver 102 may comprise a symbol separation module, a subblock interleaving module and a symbol grouping module. The symbol separation module is used to allocate the information bits and the encoded bits to 6 subblocks, which are in turn A, B, Y1, Y2, W1 and W2 described above. The subblock interleaving module is used to interleave these 6 subblocks respectively within each of the subblocks. The interleaving order is the same for each subblock. Assume that after the subblock interleaving is performed respectively to the blocks A, B, Y1, Y2, W1 and W2, the obtained bit sequences are denoted as A′, B′, Y′1, Y′2, W′1 and W′2, thenA′,B′,Y′1,Y′2,W′i,W′2=A′0,A′1, . . . ,A′N-1;B′0,B′1, . . . ,B′N-1;Y′1,1, . . . ,Y′1,N-1;Y′2,0,Y′2,1, . . . ,Y′2,N-1;W′1,0,W′1,1, . . . ,W′1,N-1;W′2,0;W′2,1, . . . ,W′2,N-1.
FIG. 2 illustrates subblock interleaving operations according to the related art. Referring to FIG. 2, a symbol separation module separates encoded bits into subblocks A, B, Y1, Y2, W1 and W2 in step 201. A subblock interleaving performs an interleaving operation to the subblocks A, B, Y1, Y2, W1 and W2 in step 202 and a symbol grouping module groups the interleaved subblocks in step 203. Herein, subblocks A and B are output by the symbol grouping module, and the two subblocks Y1 and Y2 and the two subblocks W1 and W2 are alternately output. After the symbol grouping, the output sequences are A′0, A′1, . . . , A′N-1; B′0, B′1, . . . , B′N-1; Y′1,0, Y′2,0, Y′1,1, Y′2,1, Y′2,N-1; W′1,0, W′2,0, W′1,1, W′2,1, . . . , W′1,N-1, W′2,N-1.
In the CTC of the related art, bit reliability in high order modulation is not taken into account. Here, the reliability refers to an average distance between a constellation point of which a certain mapped bit is “0” and a constellation point of which this mapped bit is “1” in a modulation constellation. The larger the distance, the greater the reliability of the mapped bit.
In a mobile communication system, in order to improve the data transmission rate without any increase of the bandwidth, an M-order Quadrature Amplitude Modulation (M-QAM) scheme may be applied. However, high order modulation is an unequal error protection modulation. For M>4, the respective bits mapped to the M-QAM symbols have different Bit Error Rates (BERs) Inner points of the constellation have less energy and thus may be subject to fading more easily. Accordingly, the bits constituting these symbols are less reliable. In contrast, the bits constituting the points outside the constellation are more reliable.
FIG. 3 illustrates reliability of bit mapping of high order modulation according to the related art. Referring to FIG. 3, the mapping order of bits is i1i2q1q2, with i1=0 and i1=1 respectively corresponding to the constellation points in the right half plane and in the left half plane, and i2=0 and i2=1 respectively corresponding to the constellation points in the middle and at the two sides of the constellation. In this way, the average distance between the constellation points where i1=1 and where i1=0 is larger than that corresponding to i2. Therefore, at the receiving end, i1 has higher reliability than i2.
FIG. 4 illustrates a constituent encoder of a ⅓ CTC encoder implementing duo binary CRSC coding according to the related art. Referring to FIG. 4, when CTC is performed, the input bit A, 401 and the input bit B, 402 are used as a set of inputs to the ⅓ CTC encoder, and the parity bits Yi and Wi embody the combined information of the information bit A, and the information bit Bi. In this type of duo binary coding, the bit A, and the bit B, should be considered as a whole and treated like a group unit. In the design of the CTC of the related art, if the bit A is mapped to a bit with high reliability, the bit B, is also mapped to a bit with high reliability. In addition, if the bit A, is mapped to a bit with low reliability, the bit B, is also mapped to a bit with low reliability. The information bits in the sequence A and the information bits in the sequence B that are simultaneously input to the constituent encoder are said to constitute a bit group. Therefore, from the perspective of the group unit (Ai, Bi), different group units have unequal bit reliability. Some group units have high reliability whereas some have low reliability.
Accordingly, there is a problem with the technique of the related art in that combining A, and B, for bit mapping is not taken into account. In addition, there is a problem with the technique of the related art in that the bit reliability of high order modulation is not taken into account during mapping.